Two Critical Next.js RCE Bugs Are Patched in 16.3.3 and 15.5.24

Vercel shipped an emergency Next.js security release on August 25, 2026, patching two critical, unauthenticated remote-code-execution vulnerabilities in 16.3.3 and 15.5.24. If you run Next.js on your own infrastructure, treat this as a same-day upgrade. If you deploy on Vercel, you are already protected.

The fixes landed a day ahead of the announced August 26 date. A second critical bug surfaced during preparation, so Vercel folded both into one release so teams upgrade only once.

What was patched

A Windows path-traversal RCE (CVE-2026-75604, CVSS 9.0). When a Next.js server runs on a Windows filesystem, a crafted request can traverse paths to unauthenticated remote code execution. It hits the Pages Router and the App Router without Cache Components across 13.4 through 15.5.24 and 16.0 through 16.3.3. There is no workaround, so Windows-hosted production apps are the most exposed and upgrading is the only fix.

An AVIF image-optimization RCE (GHSA-2xp9-vwfh-vxw4, CVSS 9.5). Next.js uses the sharp package to optimize images, and sharp leans on the libheif C library to parse AVIF files. A malicious AVIF image triggers a heap overflow in libheif (it writes 16-bit samples into an 8-bit buffer, roughly 16 KB past the allocation), reaching unauthenticated RCE when Next.js optimizes an attacker-controlled image. It covers 10.0.0 through 15.5.23 and every 16.x up to 16.3.2, and every libheif through v1.23.1. The patched releases disable AVIF optimization until the upstream fix ships.

Who is actually at risk

Exposure tracks with how you deploy, and self-hosters draw the short straw:

• Self-hosted, your own servers: exposed to both. Upgrade to 15.5.24 or 16.3.3 now. Windows hosts have no workaround for the path-traversal flaw.
• Vercel: protected from both, no upgrade required, per Vercel's August 25 changelog.
• Netlify: not affected by the Windows flaw (its functions run on Linux) and does not run the vulnerable image path, though upgrading is still advised.

No exploitation in the wild has been reported for either flaw as of publication. Both are unauthenticated and critical, so that quiet will not last.

What to do now

Upgrade the affected line:

npm install next@16.3.3   # 16.x line
npm install next@15.5.24  # 15.x line

Then rebuild your production containers, refresh your lockfile, and confirm running workloads are on the patched build. If you optimize user-supplied images and cannot upgrade this minute, disabling AVIF optimization closes the image path until you do. It is the same pattern as this month's wave of critical agent-authorization CVEs: the managed platform absorbs the blast radius, and the self-hosted deployment inherits it.